CHEMOTAXONOMY

            A. INTRODUCTION

            Chemotaxonomy or Phytotaxonomy or Chemosystematics is a branch of modern approach to taxonomy or synthetic taxonomy in which the chemical characters of plants are applied in determining the relationship among taxa of different categories.

            Some of the major classes of chemical evidence include flavonoids, alkaloids, amino acids, fatty acids, aromatic compounds, terpinoids, polysaccharides, carotenoids, etc. Studies in chemical variation have been suggested to be one of the principal growing points in the field of taxonomy. The recent growth of chemotaxonomy as a popular field of research is largely due to the development of rapid analytical techniques such as chromatography, electrophoresis, etc.

            B. ROLE OF PHYTOTAXONOMY IN SOLVINGTAXONOMIC PROBLEMS

The chemical characters may be considered under three categories namely – (i) Directly visible characters, (ii) Chemical test characters and (iii) Proteins.

1. Directly Visible Characters:- Raphides are bundle of needle shaped crystals of Calcium oxalate and occur in special sacs. General occurrence of raphides in Orchids was noted as a feature of taxonomic significance as early as 1883 by Robert Brown. Guliver reported that only three families namely – Balsaminaceae, Onagraceae and Rubiaceae can be characterized as raphide bearers. Tomlinson (1962) suggested 8 families within Scitamineae to fall into 4 natural groups – (i) Helicorniaceae, Musaceae and Strelitziaceae having raphides sacs and symmetrical gourd cells, (ii) Costaceae, Marantaceae and Zingiberaceae devoid of raphides sacs and also symmetrical gourd cells, (iii) Cannaceae lacking raphide sacs and with symmetrical gourd cells and (iv) Lowiaceae having raphides sacs and assymetrical gourd cells.

Crystals of Calcium oxalate of different shapes were also found useful in the taxonomic discrimination of the genus Allium (Jaccard and Frey, 1928).

Lapachol, a yellow powder masses can be seen in the cells of wood of the family Bignoniaceae. It is also known in Avicennia (Verbenaceae) and Bassia (Sapotaceae).

Silica occurs in a large number of dicotyledonous families. Twenty different forms of Silica have been recognized and are useful in taxonomy of Poacean members. Silica cells also occur in Arecaceae.

2. Chemical Test Characters:- Phenolics and Betalins are the two important compounds which proved to be the greatest taxonomic value. They can be extracted from plants by paper chromatography. The phenolics fall into a general class flavonoids. They are the largest group of naturally occurring phenols. Phenols are further classified as flavones, flavonones, isoflevones, isoflavonoids, flavanols, anthocyanidins, etc. These classes of phenols are known from leaves, flowers and fruits. Betalins are different from phenolic compounds by the presence of nitrogen in them. They are red and yellow pigments occurring only in 10 families of angiosperms. All these families are included traditionally in a single order Centospermae.

Oil, Fats and Waxes are the lipids together with the proteins and carbohydrates which forms the bulk of organic matter of plant tissues and therefore serves as the main source of taxonomic value. Lipids are found in all part of the plants, but occur in highest concentration in the storage organs, seeds and fruits. Plant waxes occur in cuticular layers. In flowering plants certain fatty acids like chaulmoogric acid are found in the members of Flacourtiaceae, erucic acid in Brassicaceae, petroselinic acid in Apiaceae, xymenynic acid in Santalaceae and Oleaceae, etc.

Alkaliods are present abundantly in plant families like Berberidaceae, Fabaceae, Rannunculaceae, Solanaceae, Papaveraceae, etc. The members of Fabaceae have lupin alkaloids, Solanaceae have tropane derivatives, Papaveraceae contain isoquinoline alkaloids. The families such as Annonaceae, Aristolochiaceae, Berberidaceae, Magnoliaceae, Menispermaceae and Rannunculaceae have magnoflorine and berberine alkaloids. The members of Poaceae and Asteraceae produce different types of alkaloids.

Terpenoids and Steroids are the two compounds which resemble alkaloids and glycosides. In other sense, terpenoids are regarded as components of essential oils. Over 100 monoterpenoids are known as components of essential oils. True steroids are mostly alcohol or esters.

3. Proteins:- protein occurs in large complex molecules which are universally distributed in plants in various quantities and serves as of much taxonomic value. Several examples of protein analysis and comparison in plant taxonomy are available. Some examples are mentioned below –

Desborough and Peloquin (1969) separated the soluble tuber proteins of Solanum species by disc electrophoresis. Certain bands which are consistently present in all the studies of 16 species of Solanum are of potential taxonomic value.

Johnson and Hall (1965) demonstrated phylogenetic affinities in Triticinae by protein electrophoresis.

In Magnolia and Brassica, electrophoresis has been very successful when combined with serological methods.

          In tribe Vicieae the tryptic peptides of Vicia and Lathyrus are similar suggesting the close relationship of these genera (Jackson et. al. 1967).

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